Coupling and stability of interfacial waves in liquid metal batteries

Liquid metal batteries (LMB)s are discussed today as one option for economic grid-scale energy storage, as required for the deployment of fluctuating renewable energies. Due to their completely liquid interior, LMBs are highly susceptible to various kinds of fluid dynamical instabilities, which can endanger the operational safety. This talk will primarily focus on Lorentz force-driven interfacial instabilities, as originally known from aluminum reduction cells (ARC)s and often referred to as the ‘Metal Pad Roll Instability’ (MPRI). In contrast to ARCs, predicting the interfacial dynamics in LMBs can be far more difficult since a second interface is present in the batteries. In the first part of this talk, a Potential flow analysis will be introduced describing gravity–capillary interfacial waves in three-layer fluids. As a main result, a coupling criterion is derived suggesting that coupling effects will be present in most future LMBs. Accompanying direct numerical simulation were conducted substantiating a major influence of the interfacial coupling strength to the battery’s operational safety. For closely coupled interfaces novel types of instabilities were found that cannot be described by the classical MPRI mechanism alone. In the second part, a model experiment will be presented which can mechanically induce the same multi-layer interfacial waves modes as present in ARCs and LMBs. Measurements of resonance curves and phase shifts in combination with an adapted forced wave theory model give new insights into the significance of viscous damping as well as the contact line behavior for the MPRI. Besides, the study of forced wave dynamics provides input for other research fields as for the mixing dynamics in orbitally shaken bioreactors.